Super-absorbing polymers containing tocopherol

ABSTRACT

The present invention relates to novel tocopherol-containing superabsorbents, to a process for preparing them using tocopherol-containing acid-functional monomers, especially acrylic acid, and to their use for absorbing aqueous fluids.

The present invention relates to novel tocopherol-containingsuperabsorbents, to a process for preparing them usingtocopherol-containing acid-functional monomers, especially acrylic acid,and to their use for absorbing aqueous fluids.

More particularly, the present invention relates to highly swellablehydrogels (superabsorbents) based on polyacrylic acid which containalpha-tocopherol in particular, to processes for preparing them usingalpha-tocopherol-containing acrylic acid and also to their use in ahygiene article.

Swellable hydrogel-forming addition polymers, known as superabsorbentpolymers or SAPs, are known from the prior art. They are networks offlexible hydrophilic addition polymers, which can be both ionic andnonionic in nature. They are capable of absorbing and binding aqueousfluids by forming a hydrogel and therefore are preferentially used formanufacturing tampons, diapers, sanitary napkins, incontinence articles,training pants for children, insoles and other hygiene articles for theabsorption of body fluids. Within hygiene articles, superabsorbents aregenerally positioned in an absorbent core which, as well as SAP,comprises other materials, including fibers (cellulose fibers), whichact as a kind of liquid buffer to intermediately store the spontaneouslyapplied liquid insults and are intended to ensure efficientchannelization of the body fluids in the absorbent core toward thesuperabsorbent.

The current trend in diaper design is toward ever thinner constructionshaving a reduced cellulose fiber content and an increased hydrogelcontent. The trend toward ever thinner diaper constructions hassubstantially changed the performance profile required of the waterswellable hydrophilic polymers over the years. Whereas at the start ofthe development of highly absorbent hydrogels it was initially solelythe very high swellability on which interest focused, it wassubsequently determined that the ability of the superabsorbent totransmit and distribute fluid is also of decisive importance. It hasbeen determined that superabsorbents greatly swell at the surface onwetting with liquid, so that transportation of liquid into the particleinterior is substantially compromised or completely prevented. Thistrait of superabsorbents is known as gel-blocking. The greater amount ofpolymer per unit area in the hygiene article must not cause the swollenpolymer to form a barrier layer to subsequent fluid. A product havinggood transportation properties will ensure optimal utilization of theentire hygiene article.

Superabsorbent hydrogels for use in the hygiene sector are at presentaddition polymers having a degree of neutralization in the range from 5to 80 mol %, especially in the range from 60 to 80 mol %, based on thepolymerized acid-functional monomer units.

Appropriate transportation properties are possessed for example byhydrogels having high gel strength in the swollen state. Gels lacking instrength are deformable under an applied pressure, for example pressuredue to the bodyweight of the wearer of the hygiene article, and clog thepores in the SAP/cellulose fiber absorbent and so prevent continuedabsorption of fluid. Enhanced gel strength is generally obtained througha higher degree of crosslinking, although this reduces retentionperformance. An elegant way to enhance gel strength is surfacepostcrosslinking. In this process, dried superabsorbents having anaverage crosslink density are subjected to an additional crosslinkingstep. The process is known to one skilled in the art and described inEP-A-0 349 240. Surface postcrosslinking increases the crosslink densityin the sheath of the superabsorbent particle, whereby the absorbencyunder load is raised to a higher level. Whereas the absorption capacitydecreases in the superabsorbent sheath, the core has an improvedabsorption capacity (compared to the sheath) owing to the presence ofmobile polymer chains, so that sheath construction ensures improvedfluid transmission without occurrence of the gel-blocking effect. It isperfectly desirable for the total capacity of the superabsorbent to beoccupied not spontaneously but with time delay. Since the hygienearticle is generally repeatedly insulted with urine, the absorptioncapacity of the superabsorbent should sensibly not be exhausted afterthe first disposition. Ideally, the superabsorbent will continue toabsorb the fluid speedily even on further disposition. In any case, asurge of fluid or a further second or subsequent insult may causereemergence of the fluid and hence rewet. However, when the phenomenonof gel-blocking occurs, this may cause the fluid to leak from thehygiene article. This fluid will also contain soluble components of thesuperabsorbent. Rewet is reduced by modern superabsorbents, but notcompletely prevented.

It is an object of the present invention to provide modifiedsuperabsorbents and a process for making them which on use in hygienearticles, for example, ideally cannot lead to any potential healthhazards due to the occurrence of rewet. It is another object of thepresent invention to improve the process for preparing superabsorbents.

We have found that these objects are achieved, surprisingly, by the useof highly swellable hydrogels which are polymerized from an acrylic acidwhich has been admixed with tocopherol.

The present invention accordingly provides hydrogel-forming polymerscapable of absorbing aqueous fluids and based on polyacrylate whichcontain tocopherol. Preference is given to hydrogel-forming polymerscapable of absorbing aqueous fluids where the tocopherol is distributedover the polymer. Preference is further given to hydrogel-formingpolymers capable of absorbing aqueous fluids where the tocopherol isalpha-tocopherol. Especially such hydrogel-forming polymers capable ofabsorbing aqueous fluids where the tocopherol is present in a range from10 to 1 000 ppm based on acid-functional monomers, especially acrylicacid or acrylate.

The present invention further provides processes for preparinghydrogel-forming polymers capable of absorbing aqueous fluids and basedon polymers which bear acid groups, especially based on polyacrylate, bypolymerizing acid-functional monomers, especially acrylic acid, whichcontain tocopherol. Preference is given to processes where thetocopherol is alpha-tocopherol and where tocopherol is present in arange from 10 to 1 000 ppm based on acrylic acid.

The present invention further provides hydrogel-forming polymers capableof absorbing fluid and based on acid-functional polymers, especiallypolyacrylate, obtainable by the above processes and their use forabsorbing aqueous fluids, especially in hygiene articles.

The present invention also provides acid-functional monomer, especiallyacrylic acid, containing tocopherol, especially alpha-tocopherol. Thepresent invention further provides acid-functional monomer, especiallyacrylic acid, containing tocopherol stabilizer. Tocopherol preferablymakes up at least 50 mol % of the stabilizer, particularly preferably atleast 90 mol % of the stabilizer, and especially only tocopherolstabilizer is used. The present invention also provides for the use oftocopherol to stabilize acid-functional monomers, especially acrylicacid and salts thereof.

Experimental Part

Methods of Making

a) Monomers Used

Hydrogel-forming polymers are in particular polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable grafting base, crosslinked cellulose or starchethers, crosslinked carboxymethylcellulose, partially crosslinkedpolyalkylene oxide or natural products that swell in aqueous fluids, forexample guar derivatives, alginates and carrageenans.

Suitable grafting bases can be of natural or synthetic origin. Examplesare starch, cellulose or cellulose derivatives and also otherpolysaccharides and oligosaccharides, polyvinyl alcohol, polyalkyleneoxides, especially polyethylene oxides and polypropylene oxides,polyamines, polyamides and also hydrophilic polyesters. Suitablepolyalkylene oxides have for example the formula

where

-   R¹ and R² are independently hydrogen, alkyl, alkenyl or acryl,-   X is hydrogen or methyl and-   n is an integer from 1 to 10 000.

R¹ and R² are each preferably hydrogen, (C₁-C₄)-alkyl, (C₂-C₆)-alkenylor phenyl.

Preferred hydrogel-forming polymers are crosslinked polymers having acidgroups, which are predominantly in the form of their salts, generallyalkali metal or ammonium salts. Such polymers swell particularlystrongly on contact with aqueous fluids to form gels.

Preference is given to polymers which are obtained by crosslinkingpolymerization or copolymerization of acid-functional monoethylenicallyunsaturated monomers or salts thereof. It is further possible to(co)polymerize these monomers without crosslinker and to crosslink themsubsequently.

Examples of such monomers bearing acid groups are monoethylenicallyunsaturated C₃- to C₂₅-carboxylic acids or anhydrides such as acrylicacid, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, crotonicacid, maleic acid, maleic anhydride, itaconic acid, citraconic acid,mesaconic acid, glutaconic acid, aconitic acid and fumaric acid. It isalso possible to use monoethylenically unsaturated sulfonic orphosphonic acids, for example vinylsulfonic acid, allylsulfonic acid,sulfoethyl acrylate, sulfomethyl acrylate, sulfopropyl acrylate,sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid,2-hydroxy-3-methacryloyloxypropylsulfonic acid, vinylphosphonic acid,allylphosphonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid. The monomers may be usedalone or mixed.

Preferred monomers are acrylic acid, methacrylic acid, vinylsulfonicacid, acrylamidopropanesulfonic acid or mixtures thereof, for examplemixtures of acrylic acid and methacrylic acid, mixtures of acrylic acidand acrylamidopropanesulfonic acid or mixtures of acrylic acid andvinylsulfonic acid. Particular preference is given to acrylic acid.When, in the present invention, concentrations are defined on the basisof acid-functional monomers, these concentrations relate to the totalityof monomers irrespective of whether the acid group is present inprotonated or deprotonated form. Where the concentration relates to acidgroup containing polymers, the concentration will relate to the level ofacid groups in protonated and deprotonated form.

To optimize properties, it can be sensible to use additionalmonoethylenically unsaturated compounds which do not bear an acid groupbut are copolymerizable with the monomers bearing acid groups. Suchcompounds include for example the amides and nitriles ofmonoethylenically unsaturated carboxylic acid, for example acrylamide,methacrylamide and N-vinylformamide, N-vinylacetamide,N-methyl-N-vinylacetamide, acrylonitrile and methacrylonitrile. Examplesof further suitable compounds are vinyl esters of saturated C₁- toC₄-carboxylic acids such as vinyl formate, vinyl acetate or vinylpropionate, alkyl vinyl ethers having at least 2 carbon atoms in thealkyl group, for example ethyl vinyl ether or butyl vinyl ether, estersof monoethylenically unsaturated C₃- to C₆-carboxylic acids, for exampleesters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylicacid or maleic acid, monoesters of maleic acid, for example methylhydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone orN-vinylcaprolactam, acrylic and methacrylic esters of alkoxylatedmonohydric saturated alcohols, for example of alcohols having from 10 to25 carbon atoms which have been reacted with from 2 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol, and alsomonoacrylic esters and monomethacrylic esters of polyethylene glycol orpolypropylene glycol, the molar masses (Mn) of the polyalkylene glycolsbeing up to 2 000, for example. Further suitable monomers are styreneand alkyl-substituted styrenes such as ethylstyrene ortert-butylstyrene.

These monomers without acid groups may also be used in mixture withother monomers, for example mixtures of vinyl acetate and 2-hydroxyethylacrylate in any proportion. These monomers without acid groups are addedto the reaction mixture in amounts within the range from 0 to 50% byweight, preferably less than 20% by weight.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated monomers which bear acid groups and which are optionallyconverted into their alkali metal or ammonium salts before or afterpolymerization and of 0-40% by weight, based on their total weight, ofmonoethylenically unsaturated monomers which do not bear acid groups.

Preference is given to crosslinked polymers of monoethylenicallyunsaturated C₃- to C₁₂-carboxylic acids and/or their alkali metal orammonium salts. Preference is given in particular to crosslinkedpolyacrylic acids where 5-80 mol %, preferably 60-80 mol % of the acidgroups, and in the case of acidic superabsorbents 5-30 mol %, preferably5-20 mol %, particularly preferably 5-10 mol % based on the monomerscontaining acid groups, are present as alkali metal or ammonium salts.

Possible crosslinkers include compounds containing at least twoethylenically unsaturated double bonds. Examples of compounds of thistype are N,N′-methylenebisacrylamide, polyethylene glycol diacrylatesand polyethylene glycol dimethacrylates each derived from polyethyleneglycols having a molecular weight of from 106 to 8 500, preferably from400 to 2 000, trimethylolpropane triacrylate, trimethylolpropanetrimethacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate, propylene glycol diacrylate, propylene glycoldimethacrylate, butanediol diacrylate, butanediol dimethacrylate,hexanediol diacrylate, hexanediol dimethacrylate, allyl methacrylate,diacrylates and dimethacrylates of block copolymers of ethylene oxideand propylene oxide, polyhydric alcohols, such as glycerol orpentaerythritol, doubly or more highly esterified with acrylic acid ormethacrylic acid, triallylamine, dialkyldiallylammonium halides such asdimethyldiallylammonium chloride and diethyldiallylammonium chloride,tetraallylethylenediamine, divinylbenzene, diallyl phthalate,polyethylene glycol divinyl ethers of polyethylene glycols having amolecular weight of from 106 to 4 000, trimethylolpropane diallyl ether,butanediol divinyl ether, pentaerythritol triallyl ether, reactionproducts of 1 mol of ethylene glycol diglycidyl ether or polyethyleneglycol diglycidyl ether with 2 mol of pentaerythritol triallyl ether orallyl alcohol, and/or divinylethyleneurea. Preference is given to usingwater-soluble crosslinkers, for example N,N′-methylenebisacrylamide,polyethylene glycol diacrylates and polyethylene glycol dimethacrylatesderived from addition products of from 2 to 400 mol of ethylene oxidewith 1 mol of a diol or polyol, vinyl ethers of addition products offrom 2 to 400 mol of ethylene oxide with 1 mol of a diol or polyol,ethylene glycol diacrylate, ethylene glycol dimethacrylate ortriacrylates and trimethacrylates of addition products of from 6 to 20mol of ethylene oxide with 1 mol of glycerol, pentaerythritol triallylether and/or divinylurea.

Possible crosslinkers also include compounds containing at least onepolymerizable ethylenically unsaturated group and at least one furtherfunctional group. The functional group of these crosslinkers has to becapable of reacting with the functional groups, essentially the acidgroups, of the monomers. Suitable functional groups include for examplehydroxyl, amino, epoxy and aziridino groups. Useful are for examplehydroxyalkyl esters of the abovementioned monoethylenically unsaturatedcarboxylic acids, e.g., 2-hydroxyethyl acrylate, hydroxypropyl acrylate,hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropylmethacrylate and hydroxybutyl methacrylate, allylpiperidinium bromide,N-vinylimidazoles, for example N-vinylimidazole,1-vinyl-2-methylimidazole and N-vinylimidazolines such asN-vinylimidazoline, 1-vinyl-2-methylimidazoline,1-vinyl-2-ethylimidazoline or 1-vinyl-2-propylimidazoline, which can beused in the form of the free bases, in quaternized form or as salt inthe polymerization. It is also possible to use dialkylaminoethylacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylateand diethylaminoethyl methacrylate. The basic esters are preferably usedin quaternized form or as salt. It is also possible to useglycidyl(meth)acrylate, for example.

Useful crosslinkers further include compounds containing at least twofunctional groups capable of reacting with the functional groups,essentially the acid groups, of the monomers. Suitable functional groupswere already mentioned above, i.e., hydroxyl, amino, epoxy, isocyanato,ester, amido and aziridino groups. Examples of such crosslinkers areethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, polyethylene glycol, glycerol, polyglycerol, triethanolamine,propylene glycol, polypropylene glycol, block copolymers of ethyleneoxide and propylene oxide, ethanolamine, sorbitan fatty acid esters,ethoxylated sorbitan fatty acid esters, trimethylolpropane,pentaerythritol, 1,3-butanediol, 1,4-butanediol, polyvinyl alcohol,sorbitol, starch, polyglycidyl ethers such as ethylene glycol diglycidylether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether,glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritolpolyglycidyl ether, propylene glycol diglycidyl ether and polypropyleneglycol diglycidyl ether, polyaziridine compounds such as2,2-bishydroxymethylbutanol tris[3-(1-aziridinyl)propionate],1,6-hexamethylenediethyleneurea,diphenylmethanebis-4,4′-N,N′-diethyleneurea, haloepoxy compounds such asepichlorohydrin and α-methylepifluorohydrin, polyisocyanates such as2,4-toluylene diisocyanate and hexamethylene diisocyanate, alkylenecarbonates such as 1,3-dioxolan-2-one and 4-methyl-1,3-dioxolan-2-one,also bisoxazolines and oxazolidones, polyamidoamines and also theirreaction products with epichlorohydrin, also polyquaternary amines suchas condensation products of dimethylamine with epichlorohydrin, homo-and copolymers of diallyldimethylammonium chloride and also homo- andcopolymers of dimethylaminoethyl (meth)acrylate which are optionallyquaternized with, for example, methyl chloride.

Useful crosslinkers further include multivalent metal ions capable offorming ionic crosslinks. Examples of such crosslinkers are magnesium,calcium, barium and aluminum ions. These crosslinkers are used forexample as hydroxides, carbonates or bicarbonates. Useful crosslinkersfurther include multifunctional bases likewise capable of forming ioniccrosslinks, for example polyamines or their quaternized salts. Examplesof polyamines are ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine andpolyethyleneimines and also polyamines having molar masses in each caseof up to 4 000 000.

The crosslinkers are present in the reaction mixture for example from0.001 to 20% and preferably from 0.01 to 14% by weight.

b) Free Radical Polymerization

The polymerization is initiated in the generally customary manner, bymeans of an initiator. But the polymerization may also be initiated byelectron beams acting on the polymerizable aqueous mixture. However, thepolymerization may also be initiated in the absence of initiators of theabove-mentioned kind, by the action of high energy radiation in thepresence of photoinitiators. Useful polymerization initiators includeall compounds which decompose into free radicals under thepolymerization conditions, for example peroxides, hydroperoxides,hydrogen peroxides, persulfates, azo compounds and redox catalysts. Theuse of water-soluble initiators is preferred. In some cases it isadvantageous to use mixtures of different polymerization initiators, forexample mixtures of hydrogen peroxide and sodium peroxodisulfate orpotassium peroxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate may be used in any proportion. Examples of suitableorganic peroxides are acetylacetone peroxide, methyl ethyl ketoneperoxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amylperpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate,tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butylperisononanoate, tert-butyl permaleate, tert-butyl perbenzoate,di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate,di(4-tert-butylcyclohexyl) peroxydicarbonate, dimyristylperoxydicarbonate, diacetyl peroxydicarbonate, allyl peresters, cumylperoxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate,acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxideand tert-amyl perneodecanoate. Particularly suitable polymerizationinitiators are water-soluble azo initiators, e.g.,2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis(N,N′-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyronitrile,2,2′-azobis[2-(2′-imidazolin-2-yl)propane]dihydrochloride and4,4′-azobis(4-cyanovaleric acid). The polymerization initiatorsmentioned are used in customary amounts, for example in amounts of from0.01 to 5%, preferably from 0.05 to 2.0%, by weight, based on themonomers to be polymerized.

Useful initiators also include redox catalysts. In redox catalysts, theoxidizing component is at least one of the above-specified per compoundsand the reducing component is for example ascorbic acid, glucose,sorbose, ammonium or alkali metal bisulfite, sulfite, thiosulfate,hyposulfite, pyrosulfite or sulfide, or a metal salt, such as iron(II)ions or sodium hydroxymethylsulfoxylate. The reducing component in theredox catalyst is preferably ascorbic acid or sodium sulfite. Based onthe amount of monomers used in the polymerization, from 3×10⁻⁶ to 1 mol% may be used for the reducing component of the redox catalyst systemand from 0.001 to 5.0 mol % for the oxidizing component of the redoxcatalyst, for example.

When the polymerization is initiated using high energy radiation, theinitiator used is customarily a photoinitiator. Photoinitiators includefor example α-splitters, H-abstracting systems or else azides. Examplesof such initiators are benzophenone derivatives such as Michler'sketone, phenanthrene derivatives, fluorene derivatives, anthraquinonederivatives, thioxanthone derivatives, coumarin derivatives, benzoinethers and derivatives thereof, azo compounds such as theabove-mentioned free-radical formers, substituted hexaarylbisimidazolesor acylphosphine oxides. Examples of azides are:2-(N,N-dimethylamino)ethyl 4-azidocinnamate, 2-(N,N-dimethylamino)ethyl4-azidonaphthyl ketone, 2-(N,N-dimethylamino)ethyl 4-azidobenzoate,5-azido-1-naphthyl 2′-(N,N-dimethylamino)ethyl sulfone,N-(4-sulfonylazidophenyl)maleimide, N-acetyl-4-sulfonylazidoaniline,4-sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide,p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. Photoinitiators, ifused, are customarily used in amounts of from 0.01 to 5% of the weightof the monomers to be polymerized.

The crosslinked polymers are preferably used in partially neutralizedform. The degree of neutralization is generally in the range from 5 to80%, in the case of neutral superabsorbents preferably 60-80 mol %, inthe case of acidic superabsorbents preferably in the range from 5 to 60mol %, more preferably in the range from 10 to 40 mol %, particularlypreferably in the range from 20 to 30 mol %, based on the monomerscontaining acid groups. Useful neutralizing agents include alkali metalbases or ammonia/amines. Preference is given to the use of aqueoussodium hydroxide solution, aqueous potassium hydroxide solution oraqueous lithium hydroxide solution. However, neutralization may also beeffected using sodium carbonate, sodium bicarbonate, potassium carbonateor potassium bicarbonate or other carbonates or bicarbonates or ammonia.Moreover primary, secondary and tertiary amines may be used.

Alternatively, the degree of neutralization can be set before, during orafter the polymerization in all apparatuses suitable for this purpose.The neutralization can be effected for example directly in a kneaderused for the polymerization.

Industrial processes useful for making these products include allprocesses which are customarily used to make superabsorbers, asdescribed for example in Chapter 3 of “Modern Superabsorbent PolymerTechnology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998.

Polymerization in aqueous solution is preferably conducted as a gelpolymerization. It involves 10-70% strength by weight aqueous solutionsof the monomers and optionally of a suitable grafting base beingpolymerized in the presence of a free-radical initiator by utilizing theTrommsdorff-Norrish effect.

The polymerization reaction may be carried out at from 0 to 150° C.,preferably at from 10 to 100° C., not only at atmospheric pressure butalso at superatmospheric or reduced pressure. As is customary, thepolymerization may also be conducted in a protective gas atmosphere,preferably under nitrogen.

By subsequently heating the addition polymer gels at from 50 to 130° C.,preferably at from 70 to 100° C., the quality characteristics of theaddition polymers can be further improved.

The acrylic acid used in the manufacture of superabsorbents is generallystabilized with phenolic compounds, preferably p-methoxyphenol (ModernSuperabsorbent Polymer Technology, John Wiley & Sons, Inc., 1998,Chapter 2.2.2.1 and Chapter 2.5.3). The inhibitor has to be renderedharmless prior to the free-radical polymerization. This is generallyaccomplished by inertizing with nitrogen or carbon dioxide.p-Methoxyphenol is known to require oxygen for effective stabilization.After the polymerization has ended, however, the inhibitor remains inthe product.

The acrylic acid used for manufacturing the absorbent resins generallyhas the following composition: Acrylic acid 99.5-99.95% by weight Aceticacid 0.01-0.5% by weight Propionic acid 0.001-0.1% by weight Diacrylicacid 0.005-0.2% by weight Aldehydes max. 5 ppm Inhibitor 150-250 ppm(e.g. p-methoxyphenol)

Corresponding concentrations apply to other acid-functional monomers.The acrylic acid can be produced by any desired method.

c) Surface Postcrosslinking

Hydrogel-forming polymers which are surface postcrosslinked arepreferred. Surface postcrosslinking may be carried out in a conventionalmanner using dried, ground and classified polymer particles.

Compounds capable of reacting with the functional groups of the polymersby crosslinking are applied for this purpose to the surface of thehydrogel particles, preferably in the form of an aqueous solution. Theaqueous solution may contain water-miscible organic solvents. Suitablesolvents are alcohols such as methanol, ethanol, i-propanol ethyleneglycol, propylene glycol or acetone.

The subsequent crosslinking reacts polymers which have been prepared bythe polymerization of the above-mentioned monoethylenically unsaturatedacids and optionally monoethylenically unsaturated comonomers and whichhave a molecular weight of greater than 5 000, preferably greater than50 000, with compounds which have at least two groups reactive towardacid groups. This reaction can take place at room temperature or else atelevated temperatures up to 220° C.

Suitable postcrosslinkers include for example:

-   -   di- or polyglycidyl compounds such as diglycidyl phosphonates or        ethylene glycol diglycidyl ether, bischlorohydrin ethers of        polyalkylene glycols,    -   alkoxysilyl compounds,    -   polyaziridines, aziridine compounds based on polyethers or        substituted hydrocarbons, for example bis-N-aziridinomethane,    -   polyamines or polyamidoamines and their reaction products with        epichlorohydrin,    -   polyols such as ethylene glycol, 1,2-propanediol,        1,4-butanediol, glycerol, methyltriglycol, polyethylene glycols        having an average molecular weight M_(w) of 200-10 000, di- and        polyglycerol, pentaerythritol, sorbitol, the ethoxylates of        these polyols and their esters with carboxylic acids or carbonic        acid such as ethylene carbonate or propylene carbonate,    -   carbonic acid derivatives such as urea, thiourea, guanidine,        dicyandiamide, 2-oxazolidinone and its derivatives,        bisoxazoline, polyoxazolines, di- and polyisocyanates,    -   di- and poly-N-methylol compounds such as, for example,        methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde        resins,    -   compounds having two or more blocked isocyanate groups such as,        for example, trimethylhexamethylene diisocyanate blocked with        2,2,3,6-tetramethylpiperidin-4-one.

If necessary, acidic catalysts may be added, for examplep-toluenesulfonic acid, phosphoric acid, boric acid or ammoniumdihydrogenphosphate.

Particularly suitable postcrosslinkers are di- or polyglycidyl compoundssuch as ethylene glycol diglycidyl ether, the reaction products ofpolyamidoamines with epichlorohydrin and 2-oxazolidinone.

The crosslinker solution is preferably applied by spraying with asolution of the crosslinker in conventional reaction mixers or mixingand drying equipment such as Patterson-Kelly mixers, DRAIS turbulencemixers, Lödige mixers, screw mixers, plate mixers, fluidized bed mixersand Schugi Mix, for example. The spraying of the crosslinker solutionmay be followed by a heat treatment step, preferably in a downstreamdryer, at from 80 to 230° C., preferably 80-190° C., particularlypreferably at from 100 to 160° C., for from 5 minutes to 6 hours,preferably from 10 minutes to 2 hours, particularly preferably from 10minutes to 1 hour, during which not only cracking products but alsosolvent fractions can be removed. But the drying may also take place inthe mixer itself, by heating the jacket or by blowing in a preheatedcarrier gas.

In a particularly preferred embodiment of the invention, thehydrophilicity of the particle surface of the hydrogel-forming polymeris additionally modified by formation of complexes. The formation ofcomplexes on the outer shell of the hydrogel particles is effected byspraying with solutions of divalent or more highly valent metal saltsolutions, and the metal cations can react with the acid groups of thepolymer to form complexes. Examples of divalent or more highly valentmetal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe^(2+/3+), Co²⁺,Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au^(+/3)+,preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺ and La³⁺, andparticularly preferred metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. The metalcations may be used not only alone but also mixed with each other. Ofthe metal cations mentioned, all metal salts are suitable that possessadequate solubility in the solvent to be used. Of particular suitabilityare metal salts with weakly complexing anions such as for examplechloride, nitrate and sulfate. Useful solvents for the metal saltsinclude water, alcohols, DMF, DMSO and also mixtures thereof. Particularpreference is given to water and water/alcohol mixtures such as forexample water-methanol or water-1,2-propanediol.

The spraying of the metal salt solution onto the particles of thehydrogel-forming polymer may be effected not only before but also afterthe surface postcrosslinking of the particles. In a particularlypreferred process, the spraying of the metal salt solution takes placein the same step as the spraying with the crosslinker solution, the twosolutions being sprayed in succession or simultaneously via two nozzlesor the crosslinker and metal salt solutions may be sprayed conjointlythrough a single nozzle.

Optionally, the hydrogel-forming polymers may be further modified byadmixture of finely divided inorganic solids, for example silica,alumina, titanium dioxide and iron(II) oxide, to further augment theeffects of the surface aftertreatment. Particular preference is given tothe admixture of hydrophilic silica or of alumina having an averageprimary particle size of from 4 to 50 nm and a specific surface area of50-450 m²/g. The admixture of finely divided inorganic solids preferablytakes place after the surface modification throughcrosslinking/complexing, but may also be carried out before or duringthese surface modifications.

Properties of the hydrogel-forming polymers according to the invention,of the method of making and of the acrylic acid used.

The inventive hydrogel-forming polymers capable of absorbing aqueousfluids combine a high ultimate absorption capacity with high gelstrength and permeability and also high retention.

The SFC value [in 10⁻⁷ cm³s/g] of the inventive hydrogel-formingpolymers as is measurable by the methods indicated in the descriptionpart, is preferably more than 1, especially 2, 4, 6, 8, 10, 12, 14, 16,18, 20 or higher, more preferably 22 especially 24, 26, 28, 30, 32 orhigher.

The CRC value [g/g] of the inventive hydrogel-forming polymers, as ismeasurable by the methods indicated in the description part, ispreferably more than 15, especially 16, 18, 20, 22, 24, or higher, morepreferably 25, especially 26, 27, 28, 29, 30, 31, 32 or higher.

The AUL 0.7 psi value [g/g] of the inventive hydrogel-forming polymers,as is measurable by the methods indicated in the description part, ispreferably more than 4, especially 6, 8, 10, 12, or higher, particularlypreferably 13 especially 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,26, 27, 28, or higher.

Particular preference is given to a combination of the threshold values,for example SFC with CRC, SFC with AUL, AUL with CRC, especially totriple combinations of SFC, AUL and CRC.

Polymers based on polyacrylate refers herein to such hydrogel-formingpolymers capable of absorbing aqueous fluids that contain at leastpolyacrylates. In the case of graft polymers, it is preferablypolyacrylate which has been grafted on. The fraction of acrylic acid asa hydrophilic monomer in (co)polymerized polymers or in graft(co)polymers is preferably 50% by weight or more, preferably 80% byweight or more; more preferably more than 90% by weight, 95% by weight,98% by weight, especially 99% by weight and more.

Tocopherol refers to compounds of the following formula:

-   where R¹ is H or methyl, R² is H or methyl, R³ is H or methyl and R⁴    is H or an acid radical of 1-20 carbon atoms.

Preferred radicals for R⁴ are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically acceptable carboxylic acids. The carboxylic acidscan be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R1=R2=R3=methyl,especially racemic α-tocopherol. R4 is particularly preferably H oracetyl. Particular preference is given to RRR-alpha-tocopherol.

Tocopherol is present in the polymer and in the acrylic acid monomer ina concentration which is preferably in the range from 10-1000 ppm,preferably in the range from 50-500 ppm, and especially in the rangefrom 100-300 ppm, based on acid-functional monomers, especially onacrylic acid, or on acid units in the polymer.

Preference is given to acid-functional monomer, especially acrylic acid,containing essentially only tocopherol stabilizer.

Essentially it is to be understood as meaning that tocopherol, in termsof the mol % of the stabilizers, has the highest fraction, preferablymore than 90 mol %, especially more than 95, 96, 97, 98, 99 mol %. Infree-radical polymerization, α-tocopherol is advantageous overp-methoxyphenol in that the free-radical polymerization is quicker tolight off and proceeds more smoothly. The products obtained are lessyellowish and have a Z % value of more than 70, especially more than 75,as measured using a Hunterlab LS 5100 calorimeter. The induction periodin the inventive process, involving acrylic acid for example, is ≦20sec, especially ≦15 sec, and hence distinctly shorter than on additionof MEHQ.

EP 449 913 (U.S. Pat. No. 5,159,106) describes a process for preparing(meth)acrylic esters of polyhydric alkanols by an acid-catalyzedesterification reaction in the presence of tocopherols. This is said toprevent discoloration of the esterification products.

WO 99/01410 (EP 998 437) recommends using alpha-tocopherol as apolymerization inhibitor in the production, storage and transportationof vinyl monomers, preferably acrylonitrile.

U.S. Pat. No. 5,461,124 describes the use of tocopherol in themanufacture of surgical adhesives and surgical cement.

Deployment and Use of Hydrogel-Forming Polymers

The present invention further provides for the use of the abovementionedhydrogel forming polymers in hygiene articles comprising

-   (A) a liquid pervious topsheet-   (B) a liquid impervious backsheet-   (C) a core positioned between (A) and (B) and comprising    -   (C1) 10-100% by weight of the hydrogel forming polymer according        to the invention    -   (C2) 0-90% by weight of hydrophilic fiber material-   (D) optionally a tissue layer positioned directly above and below    said core (C) and-   (E) optionally an acquisition layer positioned between (A) and (C).

Hygiene articles for the purposes of the present invention include notonly incontinence pads and incontinence briefs for adults but alsodiapers for infants.

The liquid pervious topsheet (A) is the layer which is in direct contactwith the skin of the wearer. Its material comprises customary syntheticor manufactured fibers or films of polyesters, polyolefins, rayon ornatural fibers such as cotton. In the case of non-woven materials thefibers are generally joined together by binders such as polyacrylates.Preferred materials are polyesters, rayon or blends thereof,polyethylene and polypropylene.

The liquid impervious layer (B) is generally a sheet of polyethylene orpolypropylene.

The core (C) includes not only the hydrogel forming polymer (C1) of theinvention but also hydrophilic fiber material (C2). By hydrophilic ismeant that aqueous fluids spread quickly over the fiber. The fibermaterial is usually cellulose, modified cellulose, rayon, polyester suchas polyethylene terephthlate. Particular preference is given tocellulose fibers such as pulp. The fibers generally have a diameter of1-200 μm, and preferably 10-100 μm, and also have a minimum length of 1mm.

The fraction of hydrophilic fiber material based on the total amount ofthe core is preferably 20-80% by weight and particularly preferably40-70% by weight.

Diaper construction and shape is common knowledge and described forexample in EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygienearticles are generally also described in WO 00/65084, especially atpages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502,especially pages 3-9, DE 19737434, WO 98/8439. These references and thereferences therein are hereby expressly incorporated herein.

The acidic hydrogel-forming polymers of the invention are very useful asabsorbents for water and aqueous fluids, so that they may be used withadvantage as a water retainer in market gardening, as a filter aid andparticularly as an absorbent component in hygiene articles such asdiapers, tampons or sanitary napkins.

Experimental Part

Test Methods

a) Centrifuge Retention Capacity (CRC)

This method measures the free swellability of the hydrogel in a teabag.0.2000±0.0050 g of dried hydrogel (particle size fraction 106-850 μm)are weighed into a teabag 60×85 mm in size which is subsequently sealed.The teabag is placed for 30 minutes in an excess of 0.9% by weightsodium chloride solution (at least 0.83 l of sodium chloride solution/1g of polymer powder). The teabag is then centrifuged for 3 minutes at250 g. The amount of liquid is determined by weighing back thecentrifuged teabag.

b) Absorbency Under Load (AUL) (0.7 psi)

The measuring cell for determining AUL 0.7 psi is a Plexiglass cylinder60 mm in internal diameter and 50 mm in height. Adhesively attached toits underside is a stainless steel sieve bottom having a mesh size of 36μm. The measuring cell further includes a plastic plate having adiameter of 59 mm and a weight which can be placed in the measuring celltogether with the plastic plate. The plastic plate and the weighttogether weigh 1 345 g. AUL 0.7 psi is determined by determining theweight of the empty Plexiglass cylinder and of the plastic plate andrecording it as W₀. 0.900±0.005 g of hydrogel-forming polymer (particlesize distribution 150-800 μm) is then weighed into the Plexiglasscylinder and distributed very uniformly over the stainless steel sievebottom. The plastic plate is then carefully placed in the Plexiglasscylinder, the entire unit is weighed and the weight is recorded asW_(a). The weight is then placed on the plastic plate in the Plexiglasscylinder. A ceramic filter plate 120 mm in diameter and 0 in porosity isthen placed in the middle of a Petri dish 200 mm in diameter and 30 mmin height and sufficient 0.9% by weight sodium chloride solution isintroduced for the surface of the liquid to be level with the filterplate surface without the surface of the filter plate being wetted. Around filter paper 90 mm in diameter and <20 μm in pore size (S&S 589Schwarzband from Schleicher & Schüll) is subsequently placed on theceramic plate. The Plexiglass cylinder containing hydrogel-formingpolymer is then placed with plastic plate and weight on top of thefilter paper and left there for 60 minutes. At the end of this period,the complete unit is removed from the Petri dish and subsequently theweight is removed from the Plexiglass cylinder. The Plexiglass cylindercontaining swollen hydrogel is weighed together with the plastic plateand the weight recorded as W_(b).

AUL was calculated by the following equation:AUL 0.7 psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]c) Saline Flow Conductivity (SFC)

The test method for determining SFC is described in U.S. Pat. No.5,599,335.

EXAMPLES Example of SAP Production

Starting from 1 735 g of acrylic acid, admixed with racemic α-tocopherolor MEHQ, 1 445 g of 50% aqueous sodium hydroxide solution and 2 760 g ofwater, and approximately 30% sodium acrylate solution was prepared in aconventional manner and deoxygenated with countercurrent nitrogen in astripping column in a conventional manner.

The substantially oxygen-free solution was transferred into a Werner &Pfleiderer LUK 8 trough kneader, and mixed with 7.8 g of polyethyleneglycol diacrylate and thoroughly mixed through. The reactor wasblanketed with nitrogen throughout the entire reaction time.

The initiator system, initially 32.12 g of sodium persulfate (15%solution) and then 20.79 g of ascorbic acid (0.5%), was added while thestirrer shafts were in motion. On completion of the addition, thecontents of the kneader were heated at a heating fluid temperature of74° C. The mixture began to warm up and became viscid (inductionperiod). As soon as the maximum polymerization temperature was exceeded,the heating was switched off and a supplementary polymerization wascarried out for about 15 minutes. The contents of the kneader werecooled down to 50-60° C. and discharged onto a drying sieve to form athin layer and dried in a drying cabinet at 160° C. for about 90minutes. The dried powder was subsequently adjusted to a final particlesize of from 100 to 850 μm by grinding and sieving.

Surface Crosslinking

A 5 l capacity Lödige plowshare mixer was charged with 1.8 kg ofsuperabsorbent powder prepared as per the above prescription. A solutionof 1.4 g of ethylene glycol diglycidyl ether, 59 g of water and 29 g of1,2-propanediol was sprayed onto the powder in the course of from 5 to10 min. The product is raised to a temperature of 120° C. and held atthat temperature for 60 minutes in order that the solvent may bedistilled off again. This is followed by cooling before the product isdischarged and sieved to the particle size fraction 100-850 μm.

1st Example

-   Acrylic acid with 250 ppm of racemic α-tocopherol-   Induction period≦10 sec-   End product very white (Z % value: 76)^(*))    *) Color determination (Z % values) using Hunterlab LS 5100    calorimeter

2nd Example

-   Acrylic acid with 220 ppm MEHQ-   Induction period 70 sec-   End product slightly yellow to yellow (Z % value: 63)^(*))    *) Color determination (Z % values) using Hunterlab LS 5100    calorimeter-   Induction period—α-tocopherol: typically 5-15 sec MEHQ: typically    50-150 sec

1.-12. (Cancelled)
 13. A hydrogel-forming polymer capable of absorbingan aqueous fluid and based on an acid-functional polymer containingtocopherol.
 14. The hydrogel-forming polymer of claim 1 wherein theacid-functional polymer is a polyacrylate.
 15. The hydrogel-formingpolymer of claim 1 wherein the tocopherol is distributed over thepolymer.
 16. The hydrogel-forming polymer of claim 1 wherein thetocopherol is alpha-tocopherol.
 17. The hydrogel-forming polymer ofclaim 1 wherein the tocopherol is present in an amount from 10 to 1,000ppm, based on the acid-functional monomer.
 18. A method of preparing ahydrogel-forming polymer capable of absorbing an aqueous fluid and basedon an acid-functional polymer comprising using an acid-functionalmonomer that contains tocopherol.
 19. The method of claim 18 wherein theacid-functional monomer comprises acrylic acid, and wherein the acrylicacid contains at least 50 ppm of tocopherol, and an induction period isat most 20 seconds.
 20. The method of claim 18 wherein theacid-functional monomer comprises acrylic acid and wherein the acrylicacid contains at least 50 ppm of tocopherol, and a Z % value of thehydrogel-forming polymer is greater than
 70. 21. A method of preparing ahydrogel-forming polymer capable of absorbing an aqueous fluidcomprising the use of an acid-functional monomer, wherein theacid-functional monomer contains tocopherol.
 22. The method of claim 21wherein the tocopherol comprises alpha-tocopherol.
 23. The method ofclaim 21 wherein the acid-functional monomer contains tocopherol as asole stabilizer.
 24. The method of claim 21 wherein the acid-functionalmonomer comprises acrylic acid or an acrylate salt.
 25. Ahydrogel-forming polymer capable of absorbing aqueous fluids prepared bythe method of claim
 21. 26. A method of absorbing an aqueous fluidcomprising contacting the aqueous fluid with a hydrogel-forming polymerof claim
 1. 27. A hygiene article comprising a hydrogel-forming polymerof claim 1.